In response to concerns about dwindling petroleum supplies, manufacturers of automotive vehicles are developing hybrid electric vehicles, electric vehicles, and plug-in hybrid electric vehicles (PHEVs), the latter two deriving all or some of their power from an electrical grid during a charging operation. Such vehicles are equipped with a battery charger connected to a receptacle into which an external power supply cord can be plugged for charging an on-board battery pack. It is desirable to prevent driveaway of the vehicle when such an external power supply cord is connected to the vehicle. An electric-vehicle battery-charger interlock system that disallows the electric motor from propelling the vehicle while charging is disclosed in U.S. Pat. No. 3,898,547. However, such a system does not prevent vehicle movement due to rolling under the action of gravity or due to propulsion by a secondary power source, such as an internal combustion engine provided in a PHEV.